zlib.compress releases the GIL so can be multithreaded.
This patch adds a context manager to FBXElem that temporarily enables
multithreading of the compression of added arrays, by using the recently
added utility to schedule CPU-bound tasks to run on separate threads.
On my Ryzen 7 7800x, exporting non-animated rigged humanoid models
typically results in about a 1.2 to 1.4 times faster export.
Exporting only simple geometry, such as many subdivided default cubes,
can be about 2 to 3 times faster.
No changes are expected to the contents of exported files.
